The purification of thermally sensitive, unstable, or reactive compounds such as acrylic acid is typically attempted by continuous vacuum distillation or other vapor-liquid separation processes utilizing low pressure drop trays.
The known distillation columns (column(s)) commonly employed in the separation of thermally unstable and reactive compounds experience hydraulic instability, oscillation and unpredictable behavior. Column trays run dry or have dry spots. These columns and trays experience polymer deposition, fluid bypassing, poor hydraulics, poor mixing, flow channeling and flow maldistribution, liquid weeping and poor liquid-vapor interaction. Any one, or a combination, of these factors can result in reduced tray efficiencies and poor column performance.
The presence of polymer in separations processes is a significant problem. Polymer can be present or accumulate on column trays, or anywhere throughout the column. Vapor phase condensation, or liquid-phase polymerization has an equal chance of seeding on any internal column surface. Large gradients in polymer volume may be seen across or down a column's physical profile. Polymer may migrate throughout the column and associated equipment. As a result stream strainers and pumps plug with polymer. The columns must be frequently cleaned. Maintenance and personnel costs are burdensome and run times are short.
In traditional distillation processes for thermally sensitive materials, high temperatures and/or high per tray pressure drops have been determined to increase unwanted polymerization and create production difficulties including loss of efficiency, reduced capacity, damage to equipment, high maintenance and increased energy costs. Inhibitors can be employed to reduce polymerization, however the cost of inhibitors is significant and reduces product purity. In view of these problems, it is common for manufacturers to employ smaller diameter columns or multiple columns. Such approaches are burdensome to operate and expensive to build.
The aforementioned hydraulic and polymer based problems lower tray and column efficiency and prevent separations processes from meeting purity specifications. They also create process control difficulties, result in unreliable sensor data and prevent accurate process simulation.
The purification processes utilized by manufacturers of acrylic acid (an industrially important compound which is reactive and thermally unstable) experience problems including, but not limited, to those set forth above when production capacity is high.